1. Field of the Invention
The present invention relates, in general, to light measuring circuitry for a camera and, in particular, to a temperature compensation method, suited for monolithically integrating the light measuring circuitry. In said circuitry, the photo current from a photoelectric transducing element is amplified by an operational amplifier having logarithmic compression characteristics and then expanded expotentially by means of a transistor and so on.
2. Description of the Prior Art
The technical background in this field will be explained with reference to FIG. 1. FIG. 1 shows an example of light measuring circuitry for a camera. 1 is an operational amplifier (hereinafter called OP1) having a high input impedance, for example, having MOS-FET as the input step. 2 is a photoelectric transducing element, such as silicon photo diode, connected between the non-inverting input and the inverting input of OP1 so as to produce a current I.sub.SPD proportional to the brightness of an object to be photographed. 3 is a logarithmic compression diode connected between the output and the inverting input of OP1 so as to constitute a negative feedback circuit. 4 is a reference voltage source having an output voltage Vref for biasing the noninverting input terminal of OP1. 5 is an expansion transistor whose base is connected to the output of OP1 and whose emitter is grounded. 6 is a timing capacitor having a capacitance C and which is connected between the collector of the expansion transistor and a power source or a constant potential 7. 8 is a count switch which is normally closed and opened in synchronization with the initial count timing when a time inversely proportional to an incident light on the light sensing element 2 should be obtained and 9 is a comparator, one input of which is connected to an almost constant potential V.sub.TH from 7 independent of temperature, and whose other input is connected to the collector of expansion transistor 5. In this schematic, the output voltage OP1 OUT of OP1 is represented with the characteristics of the ideal operational amplifier and the diode currentvoltage characteristics as follows: ##EQU1## Hereby, q: charge per electron (coulomb)
k: Boltzmann constant PA1 Is: backward saturation current of the compression diode PA1 T: ambient temperature expressed with absolute temperature (.degree.k) PA1 Vref: reference voltage PA1 I.sub.SPD : photo current of the light sensing element
Suppose that the current flowing through expansion transistor 5 is Ic and H.sub.FE (the current gain) of transistor 5 is so large that the difference between Ic and Ie (emitter current) can be neglected. Then the base potential V.sub.BASE is represented as follows: ##EQU2##
Hereby, Is is the backward saturation current at the base-emitter junction of expansion transistor 5. Is can be expected to be the same value and to have the same characteristics as Is in equation (1) by fabricating both compression diode 3 and expansion transistor 5 in the same monolithical way. Namely, Is in (1) becomes equal to Is in (2). Since OP1 OUT in the equation (1) is equal to V.sub.BASE in the equation (2), then ##EQU3##
Thus, EQU Ic/I.sub.SPD =EXP(q.times.Vref/kT) (3)
On the other hand, the time interval after the closed count switch 8 is opened until the timing capacitor 6 is charged, with the expansion current Ic, to V.sub.TH is represented as follows: ##EQU4##
Substitution of (3) in (4) gives ##EQU5##
As is apparent from the above, in order for t to be independent of temperature and inversely proportional to the photo current I.sub.SPD, it is necessary for the reference voltage Vref to be proportional to the absolute temperature so as to be able to eliminate the termperature term T. Even if Vref=0 in (5), t is independent of temperature and inversely proportional to I.sub.SPD. However, I.sub.SPD is generally very small, so it is ordinarily necessary to obtain Ic which is amplified from I.sub.SPD by 1000 to 2000 times. Thus, the reference voltage Vref is often selected to be about 200 mV in order to obtain such a high amplification factor.
Consequently, in a circuit in which the light measuring or exposure control is carried out with the voltage obtained when the short circuit current of the photodiode flows through the compression diode, it is necessary to devise many means for temperature compensation which requires a voltage proportional to the absolute temperature. Whereby, for example, a resistor having a resistance almost proportional to the absolute temperature can be used. However, in this case, the cost of such a resistor is not only high, in general, but it has poor stability or linearity and then high accuracy cannot be guaranteed, which is inconvenient. In this case, another resistor with a low temperature coefficient is often needed in addition to the special resistor mentioned above, which is not suitable to monolithically integrable semiconductor circuits. Further, as is disclosed in U.S. Pat. No. 4,072,962, a constant current source whose output is proportional to the absolute temperature is designed in a manner such that a voltage proportional to the absolute temperature is obtained by letting the current flow through a resistor. Even in this case, not only does the circuit become complicated, but theoretically, resistors without temperature coefficients seem to be needed as external parts around the monolithic integrated circuit, which is also inconvenient.